Mounts two gocryptfs filesystems, "ping" and "pong" and moves the
linux-3.0 kernel tree back and forth between them.
When called as "pingpong-rsync.bash" it uses "rsync --remove-source-files"
for moving the files, otherwise plain "mv".
Running these tests from integration_tests' TestMain() was awkward
because they were run twice with unchanged settings.
integration_tests tests everything with OpenSSL and with native
Go crypto, but this does not take affect for the example filesystems.
To make this work, test_helpers is also split into its own package.
